10 / 2021-04-01 15:45:09

Global hydrogen particle balance to analyze recycling and particle confinement at Wendelstein 7-X

Wendelstein,Recycling，,confinement,pwi

A single-reservoir particle balance for the main ion species hydrogen has been established at Wendelstein 7-X. This has enabled the quantitative characterization of the particle sources and sinks in the standard island divertor configuration for the first time. Findings from attached to first results for detached divertor scenarios with the uncooled graphite divertor after boronization are presented. Fueling efficiencies, flux balances and source locations were measured and used to infer the total particle confinement time τ_P in gas injection fueled plasmas, without pellets or neutral beam injection. Perturbative gas injection experiments served to measure the effective particle confinement time τ_P^*. Combining both confinement times provides access to the overall recycling coefficient R. Hydrogen particle inventories have been addressed and the knowledge of particle sources and sinks provides insight into the capability of the magnetic island topology to control exhaust features.

Measurements of hydrogen fueling efficiencies revealed that for upstream gas fueling, located at the inner midplane of the machine, 28 % of the injected particles fuel the confined plasma, while downstream fueling through the island at the horizontal divertor target was sensitive to the precise fueling location and varied between 12 % and 44 %. The fueling efficiency of helium was overall about 60 % higher at the respective locations.

The recycling flux was found to govern the particle source. It was shown that in attached scenarios, 76 % of recycled particles re-enter the plasma from the divertor, while the remainder of the total recycling flux ionized above the baffle (9 %), heat shield (11 %) and steel panels (4 %). τ_P values in the range of 100 - 120 ms, depending on the exact density, were extracted for these discharges.

An increase of the island size and shape through a control coil current, resulted in no change of τ_P but doubled τ_P^*, indicating the feasibility of the control coils as an actuator for stable divertor operation. First assessment of detached divertor conditions shows that the fueled particle flux was fully balanced by the pumped flux with a constant wall source over seven seconds of detached plasma state. The particle confinement time τ_P doubled to levels of around 250 ms for this time. This is a result of a decrease of the overall recycling flux at an increased density level in equilibrium between sources and sinks.



Acknowledgements: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was funded in part by the Department of Energy under grant DE-SC0014210.